1. Field of the Invention
The present invention relates to a three-state electromagnet and to a control circuit for said electromagnet.
2. Description of the Prior Art
Three-state electromagnets are useful for example for controlling an equipment unit or device between a quiescent state and two different operating states. By way of example, an electromagnet of this type can be mounted in a contactor for putting a three-phase motor in forward motion, in reverse motion or in the stationary state. A contactor of this type comprises contacts of the changeover type which are also capable of assuming an intermediate position between the two end positions. In order to prevent any short-circuiting of the source during the passage of electric arcs across the contacts, it must be ensured that the speed of changeover of the contacts is not too high.
A known form of polarized contactor having three stable positions has already been disclosed in German patent DE A1 No. 31 38 265 and in French patent FR-A No. 2 532 107. This contactor comprises an electromagnet of the bistable type having a permanent magnet and a restoring spring which is totally inactive in the central position of the armature but produces an abrupt variation as soon as the armature moves away from its central position. The permanent magnet ensures stability of each end position in spite of the opposing action of the spring.
This known contactor is thus tristable. However, applications involving the use of monostable contactors are more common. Furthermore, in order to leave an end position, the coil must be excited in a direction opposite to that of the preceding excitation. At this stage, however, the armature is liable to pass beyond the stable central position and to move right up to the other end position. Instead of stopping, the controlled motor will rotate in the opposite direction, which is liable to be highly dangerous. It is in fact known that ampere-turns cannot easily be regulated by reason of the variations in voltage and resistance which are caused by heating. Furthermore, the force produced by the magnets varies with the temperature and the force exerted by the contact springs decreases with wear of the contacts.
In order to overcome this deficiency, the document of the prior art proposes to excite the windings simultaneously in opposite directions. Their total effect is then only the effect of leakages caused by the different positions of the windings, thus resulting in low efficiency. Furthermore, switching or changeover of the windings is difficult to carry out in practice. In addition, a stable central position cannot readily be obtained by means of a spring and makes it necessary to take practical precautions which entail high capital expenditure. By way of example, reference can be made in this connection to the book entitled "La Telegraphie et le Telex" by D. Faugeras, published in 1962 by Eyrolles, page 194: three-position relay.
Means for making the contactor monostable are admittedly proposed in one of the cited documents but these means are insufficient in actual practice.
As disclosed in patent EP - A No. 86 121, there is also known an electromagnet having two moving systems provided with permanent magnets and capable of relative displacement while defining four air-gaps between them. The permanent magnets are in series with each other at one end position which is therefore stable and are in opposition to each other at the other end position which is therefore unstable. An electromagnet of this type is in fact monostable without artifice. But if it is employed in a contactor, the electromagnet is capable of placing the power contacts only in two different positions and not three.
French patent FR No. 2 554 957 (not published on the priority date of the present Application) describes an electromagnet which is of the same type (with two positions and two permanent magnets) but is bistable.
Another known device disclosed in U.S. Pat. No. 2,872,546 is a three-position monostable electromagnet in which a rotating permanent magnet is mounted between two fixed magnets, the midpoint of each fixed magnet being joined to one end of a fixed yoke. When no excitation is applied, the moving magnet assumes an intermediate position in which its north pole is at equal distance from the north poles of the fixed magnets and its south pole is located at equal distance from the south poles of the fixed magnets. The moving magnet pivots either in one direction or in the other, depending on the direction of excitation of a coil which surrounds the yoke. However, this electromagnet is inefficient since the only practical effect of the coils is to suppress certain repulsive forces while allowing others to remain.
It is in fact known that, repulsive forces are weaker than attractive forces in closed air-gaps.
The object of the invention is thus to propose a three-position monostable electromagnet which develops high magnetic forces in the "work" positions, which is not liable to move from one end position to the other when it has been operated solely for a return to the central or intermediate position, which does not require any costly modification such as an increase in range of travel or in inertia of the moving system and which is not liable to change-over too rapidly from one end position to the other since this would entail the risk of a short-circuit.